Aeroelastic Behavior of Bird-Damaged Fan Blades Using a Coupled CFD/CSD Framework.

Abstract

Bird strike is a growing concern in the design of modern high-bypass turbofan engines. Predicting the aeroelastic behavior of a bird-damaged fan blade represents a significant barrier in the development of improved-efficiency turbofan engines. In this dissertation, the aeroelastic response of a bird-damaged fan stage at the inlet of a high-bypass ratio turbofan engine is examined using a combined computational fluid dynamics (CFD) and computational structural dynamics framework. The damaged fan contains a sector of 5 damaged blades obtained from accurate numerical simulation of the bird impact. Unsteady aerodynamic and aeroelastic response calculations are performed at 100%, 75%, and 60% take-off thrust conditions to investigate the role of engine speed on the fan response.

A CFD-based aerodynamic model is utilized to perform the steady and unsteady aerodynamic calculations for the undamaged and damaged fan. An automated mesh deformation scheme using radial basis function interpolation is developed to generate a high quality computational mesh for the damaged geometry. The steady calculations of the damaged fan predict significant flow loss with the fan operating near stall where unsteadiness is significant. The unsteady calculations of the damaged fan exhibit a periodic behavior dominated by the progression and regression of a stall cell that produces significant unsteady aerodynamic loads on the fan blades.

The aerodynamic model is coupled with a finite element based structural dynamics model to perform both one-way forced response and fully-coupled aeroelastic response calculations. The undamaged blades downstream of the damaged sector exhibit the greatest structural response that is dominated by the first bending mode. When compared to the forced response calculation, the aeroelastic response includes increased participation of the higher structural modes, especially for the damaged blades, that grow in time or exhibit beating. Examination of the work exerted by the aerodynamic forces suggests that the growth in amplitude of the higher modes may result from aeroelastic instability.

This study provides substantial contribution toward improved understanding of the bird strike problem and demonstrates the feasibility of performing aeroelastic response calculations of a bird-damaged fan. The results illustrate the importance of aeroelastic coupling when predicting the post bird strike fan response.